1. Field of the Invention
This invention relates generally to receiver circuit architecture in a portable communication device. More particularly, the invention relates to DC offset detection and cancellation in a receiver.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, third generation portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations thereof and the access format is TDMA.
In Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology.
Furthermore, the need for higher data transmission capability has given rise to enhancing the GSM standard. This relatively new standard is referred to as enhanced data rates for GSM evolution, also referred to as “EDGE.” The EDGE standard uses burst-type transmission and a combination of phase and amplitude modulation to increase the amount of data that can be transmitted.
One of the advances in portable communication technology is the move toward the implementation of a low intermediate frequency (IF) receiver and a direct conversion receiver (DCR). A low IF receiver converts a radio frequency (RF) signal to an intermediate frequency that is lower than the IF of a convention receiver. A direct conversion receiver downconverts a radio frequency (RF) received signal directly to baseband (DC) without first converting the RF signal to an intermediate frequency (IF). One of the benefits of a direct conversion receiver is the elimination of costly filter components used in systems that employ an intermediate frequency conversion.
When implementing a low IF or a direct conversion receiver, there is typically some amount of offset (referred to as “DC offset”) that appears on the downconverted signal. The DC offset occurs primarily due to self-mixing that can occur with the local oscillator (LO) signal, the radio frequency (RF) signal or interfering signals in the receiver but can also be due to other sources such as circuit bias voltages. Self-mixing among the LO, RF and interfering signals, as well as reflection at the antenna, temperature variation and LO leakage result in dynamic DC offset. For instance, where DC offset is generated by the self-mixing of an LO signal the level of the DC offset will depend on the leakage path, which transfers a portion of the LO signal to the mixer input. This leakage path generally is an unintended, parasitic parameter of the receiver circuit and is dependent on such parameters as manufacturing process variation, device temperature, LO frequency and LO signal level. Many techniques have been proposed to detect and minimize DC offset. For example, it is possible to remove some of the unwanted DC in the analog domain and some in the digital domain (i.e. in the baseband digital receiver). Unfortunately, this solution fails to remove sufficient DC offset in the EDGE communication environment because EDGE uses a burst-type transmission methodology and requires a relatively high signal-to-noise ratio to support the desired data rates. Current techniques to cancel DC offset leave a residual error, thus creating a limiting factor on EDGE receiver performance. The residual error after DC cancellation is directly related to estimating the mean DC associated with a set of relatively noisy data. For purposes of DC offset estimation, the mean is the DC offset and the desired signal is treated as noise.
One prior technique attempts to reduce the level of the DC in the analog domain, but is difficult to implement successfully because the dynamic nature of the DC offset makes it difficult to accurately estimate and thus to remove in the analog domain.
Another possible technique is to estimate the DC offset in the digital domain after the signal has been digitized and sent from the RF receiver to the baseband digital signal processor (DSP) device. Unfortunately, in typical wireless communication systems the sampling rate of the received signal sent to the DSP is reduced from that used within the RF receiver in order to minimize the complexity of the baseband DSP hardware requirements. This means that, in a burst-type transmission system such as EDGE/GSM, the total number of samples sent from the RF receiver to the DSP device is typically too small to allow the burst mean, i.e. the DC offset of the burst, to be estimated to the desired level of accuracy. This is a direct consequence of the Cramer-Rao bound, which states that the accuracy of any technique to measure the mean of a set of noisy data samples is limited by the number of samples in the set. Furthermore this technique requires an expenditure of digital signal processing (DSP) resources in the baseband device. Further, this technique increases the power consumption of the device.
Therefore, it would be desirable to provide DC offset detection and cancellation in a receiver operating in a communication system that can be used for EDGE and other burst-type communication systems.